An inkjet printer includes one or more ink-filled pens that are mounted to a carriage in the printer body. Normally, the carriage is scanned across the width of the printer as paper or other print media is advanced through the printer. Each ink-filled pen includes a printhead that is driven to expel droplets of ink through an array of nozzles in the printhead toward the paper in the printer. The timing and nominal trajectory of the droplets are controlled to generate the desired text or image output and its associated quality.
As the sheet of print media is advanced through the printer, it must be secured so that high-resolution printing can occur. One method of holding the sheet is to direct it against an outside surface of a moving carrier such as perforated drum. Suction is applied to the inside surface of the carrier for holding the sheet against the moving carrier. The carrier is arranged to move the sheet into and out of a location adjacent to the pens that apply the ink to the sheet.
It is important to apply the proper level of suction to a system like the one just described. The suction, or vacuum pressure (here the term "vacuum" is used in the sense of a pressure less than ambient), must be applied at a level sufficient for ensuring that the sheet of print media remains in contact with the carrier. For example, should the edges of the sheet lift from the carrier as a result of too little vacuum pressure, there is a likelihood that the pen will collide with the edge, which is quite undesirable. Also, the vacuum pressure level must be high enough to hold the sheet flat, to eliminate wrinkling or cockling of the sheet during printing.
If the vacuum pressure level is too high, the surface of the sheet may become deformed in the vicinity of the perforations. As a result, the ink droplets will not strike the surface of the sheet as intended, and print quality will suffer. Also, power is wasted if the vacuum level is unnecessarily high.
Moreover, when liquid ink is applied to the sheet, it is important to ensure that the vacuum pressure level is not so high as to draw the ink completely through the sheet, such that the ink appears on the other side as an undesirable effect known as "strike through."
The foregoing considerations concerning vacuum levels are complicated by differences in the physical characteristics of the variety of print media that can be handled by modern printers. The print media can be thin, relatively lightweight cut paper, relatively thick or stiff media known as transparencies, heavy photo stock, etc. Also, media having the same thickness will not necessarily deform by the same amount for a given vacuum pressure level. For example, a sheet of transparency type media having a given thickness will not deform by the same amount as a sheet of paper having the same thickness. In short, one level of vacuum pressure will not be appropriate for the wide variety of print media available to a user.
The present invention is directed to a system for controlling or regulating the vacuum hold pressure in a printer based upon the sensed stiffness of the print media that is directed through the printer.
In one preferred embodiment of the invention, the deflection of the print media is sensed before or as the media reaches the carrier. The vacuum pressure level is regulated in response to this deflection measure, thereby to have applied to that particular media a level of vacuum pressure that is best (remove cockle, avoid strike through, etc.) for that media.